I. Field of the Invention
The present disclosure relates generally to the fields of chemistry and materials science. More particularly, it concerns metal-organic frameworks, compositions thereof and methods use thereof, including for separating gas molecules such as ethylene and acetylene.
II. Description of Related Art
Microporous metal-organic frameworks (MOFs) have emerged as a new type of porous materials for gas storage, separation, sensing and heterogeneous catalysis. The tunable pores and the immobilized functional sites within such microporous MOFs have enabled them to direct specific recognition of certain molecules based upon size and functionality.
Ethylene is an industrially utilized raw chemical with over 100 million tons produced in 2006 (Sundaram et al., 1995). Ethylene is used to produce polymers and other useful chemicals (Sundaram et al., 1995). During the production of ethylene through the cracking of ethane, propane and heavier hydrocarbons, a small amount of acetylene as an impurity of about 1% was also generated. Acetylene in the ethylene feed should be reduced to an acceptable level because acetylene has a deleterious effect on end products of ethylene. For example, acetylene can poison catalysts during ethylene polymerization and thus affect the quality of the resulting polyethylene. Furthermore, acetylene can form solid metal acetylides, which can block the fluid stream and lead to explosion (Molero et al., 1999).
Extensive efforts have been pursued to remove acetylene from ethylene/acetylene mixtures (Studt, et al., 2008 and Lewis, 1974). In the petro-chemical industry, current commercial approaches include partial hydrogenation of acetylene into ethylene over a noble metal catalyst such as a supported Pd catalyst and solvent extraction of cracked olefins using an organic solvent such as DMF and acetone. Both of which have some drawbacks: the former process suffers from the need of noble-metal catalyst and the loss of olefins due to the over hydrogenation to paraffins, while the latter wastes much amount of solvents. Porous materials through selective adsorption separation of acetylene over ethylene might provide the alternative cost and energy efficient approach for this industrially useful but difficult task, including removing acetylene from ethylene in amounts as low as 1%.
Among diverse porous materials, the emerging microporous metal-organic frameworks (MOFs) are useful in gas separations including the removal of acetylene from ethylene/acetylene mixtures. The pores within microporous MOFs can be tuned to enforce their size selective sieving effects while their pore surfaces can be readily functionalized to induce their preferential interactions with specific gas molecules (Furukawa, et al., 2013; Sato, et al., 2014; Zhao, et al., 2013; An, et al., 2012; Xiang, et al., 2012; Li, et al., 2013; Vaidhyanathan, et al., 2010; Chen, et al., 2010; Lin et al., 2012; Férey, et al., 2005; Zhao, et al., 2004; Farha, et al., 2010; Mohideen, et al., 2011; Zhang, et al., 2012; Zhang & Chen, 2009; Li, et al., 2012; Burd, et al., 2012; Nugent, et al., 2013; Shekhah, et al., 2014; Li, et al., 2014; Ma, et al., 2009, Lan, et al., 2011; Motkuri, et al., 2014; Li., et al., 2014; Li, et al., 2014; Chen, et al., 2006; Guo, et al., 2011 and He, et al., 2012). MOFs for the separation of C2H2/C2H4 have not been fully explored. An initial microporous MOF for this separation was developed in 2011 (Xiang, et al., 2011). The micropores have been further developed through the interplay of metalloligands and organic linkers, and thus to optimize the separation selectivities. Although the separation selectivities of these MOFs for the separation of C2H2/C2H4 are high because of their high sieving effects; their narrow pores have also limited their acetylene uptakes, which eventually affect their overall performance for separation of C2H2/C2H4, as clearly demonstrated in the simulated breakthrough curves (Das, et al., 2012). Further development led to the discovery of MOF-74 series for C2H2/C2H4 separations (Bloch, et al., 2012 and He, et al., 2012). This series of MOFs have high densities of open metal sites which can enforce their acetylene uptakes (Xiang, et al., 2009 and Xiang, et al., 2010), but their pores are too large to introduce size sieving effects. Furthermore, the open metal sites have strong interactions with ethylene molecules as well, so MOF-74 series have systematically low selectivities for C2H2/C2H4 separation. The MOFs for C2H2/C2H4 separation are those with high C2H2/C2H4 sieving effects but without sacrificing acetylene uptakes. There has been some progress on microporous MOFs for C2H2/C2H4 separation; however, their pore structures still cannot meet the necessary selectivity criteria and their separation performances are comparable to earlier MOFs (Yang, et al., 2015 and Wen, et al., 2015). As such MOFs that exhibit selective and effective removal of acetylene from ethylene/acetylene mixtures containing mixtures containing as little as 1% acetylene are needed.